Contribution of Nutrient and Organic Matter Sources to the Development of Periodic Hypoxia in a Tributary Estuary

The seasonal formation of periodic hypoxia within tributary estuaries is influenced by multiple nutrient and organic matter sources, both autochthonous and allochthonous. In contrast to lakes and flowing rivers, allochthonous inputs to estuaries include those from near-field sources in the watershed as well as far-field sources from adjacent marine systems that contribute nutrients, organic matter, and hypoxic water via estuarine circulation. This study utilized a reduced-complexity eutrophication model to quantify the contribution of individual sources to hypoxia in the York River estuary (YRE), and assess the reductions necessary to mitigate hypoxic conditions. Results indicate that portions of the oligohaline YRE are influenced by organic loading from the watershed, while the lower mesohaline region is influenced by both internal phytoplankton production and watershed loads. In the high mesohaline, watershed organic matter and internal phytoplankton production play an equally important role during the spring; however, in summer and fall oxygen concentrations appear to respond strongly to dissolved organic carbon (DOC) entering via advection from Chesapeake Bay (CB). The polyhaline region, which is frequently the site of reoccurring hypoxia, responded primarily to advected DOC from CB. Results indicate that different regions of the YRE may require different management strategies, and highlight the strong relationship between periodic hypoxia in the lower YRE and advection of labile organic matter from CB, a far-field input that requires a regional management approach. A cross-system comparison of nutrient budgets indicates that far-field sources can comprise a large portion of total system inputs in numerous tributary estuaries.